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Utpal Mukhopadhyay

Publications and source records attributed to Utpal Mukhopadhyay.

At least 19 recordsLinked to original sources
arXiv2021

Compact stellar models in modified gravity

In the present investigation compact stellar models are dealt with in the framework of the modified gravity theory, specifically of $f(\mathbb{T},\mathcal{T})$ type. We have considered that the compact objects are following a spherically symmetric static metric and obtained the Einstein field equations in the spacetime of $f(\mathbb{T},\mathcal{T})$. To make the Einstein equations solvable we employ the methodology of conformal Killing vectors. Thereafter by using the MIT bag equation of state to the compact stars, considering that the stars are formed by strange quark, we find the solutions set. The solutions are examined via several physical tastings which exhibit viability of the model.

gr-qc↗
arXiv2015

A Dark Energy Model in Kaluza-Klein Cosmology

We study a dynamic $Λ$ model with varying gravitational constant $G$ under the Kaluza-Klein cosmology. Physical features and the limitations of the present model have been explored and discussed. Solutions are found mostly in accordance with the observed features of the accelerating universe. Interestingly, signature flipping of the deceleration parameter is noticed and the present age of the Universe is also attainable under certain stringent conditions. We find that the time variation of gravitational constant is not permitted without vintage $Λ$.

physics.gen-ph↗
arXiv2014

N.C. Rana: The Life of a `Comet' in the Astrophysical World

Narayan Chandra Rana, a person with extraordinary potential from a remote village of Bengal, India, came into the limelight of the international scientific world through his exceptional talent, zeal and courage. In his very short life-span, he excelled not only into various branches of astrophysics, but also took a leading role in science popularization, text book writing etc. In this paper, life and works of that budding scientist of India have been discussed from multifarious viewpoints.

physics.hist-ph↗
arXiv2013

Scenario of Accelerating Universe from the Phenomenological Λ- Models

Dark matter, the major component of the matter content of the Universe, played a significant role at early stages during structure formation. But at present the Universe is dark energy dominated as well as accelerating. Here, the presence of dark energy has been established by including a time-dependent $Λ$ term in the Einstein's field equations. This model is compatible with the idea of an accelerating Universe so far as the value of the deceleration parameter is concerned. Possibility of a change in sign of the deceleration parameter is also discussed. The impact of considering the speed of light as variable in the field equations has also been investigated by using a well known time-dependent $Λ$ model.

astro-ph↗
arXiv2013

Oscillatory Universe, dark energy and general relativity

The concept of oscillatory Universe appears to be realistic and buried in the dynamic dark energy equation of state. We explore its evolutionary history under the frame work of general relativity. We observe that oscillations do not go unnoticed with such an equation of state and that their effects persist later on in cosmic evolution. The `classical' general relativity seems to retain the past history of oscillatory Universe in the form of increasing scale factor as the classical thermodynamics retains this history in the form of increasing cosmological entropy.

physics.gen-ph↗
arXiv2012

Indian Amateur Astronomer R. G. Chandra: A unique AAVSO member

In this Note, which is a sequel on the Indian amateur astronomer R.G. Chandra [Biswas2011a, Biswas2011b], we have presented some documents to reveal his three-decades relationship with the American Association for Variable Stars Observers and other connection to American astronomical societies. We have given a short account of his observations on different category, viz. (i) Variable stars, (ii) Nova, (iii) Meteors and (iv) Andromedae. His responsibilities and recognitions are also discussed.

physics.hist-ph↗
arXiv2010

Phenomenology of $Λ$-CDM model: a possibility of accelerating Universe with positive pressure

Among various phenomenological $Λ$ models, a time-dependent model $\dot Λ\sim H^3$ is selected here to investigate the $Λ$-CDM cosmology. Using this model the expressions for the time-dependent equation of state parameter $ω$ and other physical parameters are derived. It is shown that in $H^3$ model accelerated expansion of the Universe takes place at negative energy density, but with a positive pressure. It has also been possible to obtain the change of sign of the deceleration parameter $q$ during cosmic evolution.

gr-qc↗
arXiv2010

Variable Equation of State for Generalized Dark Energy Model

We present a model for the present accelerating Universe and focus on the different important physical variables involved in the model under the phenomenological assumption $Λ\propto H^2$ with a prescription for equation of state parameter in the form $ω(t)=ω_0+\frac{ω_1τ}{t^2}$, where $ω_0$ and $ω_1$ are two constants and $τ$ is a parameter having dimension of time $t^2$. General expressions for the density parameter $Ω$ and deceleration parameter $q$ are obtained which under specific bound reproduce some of the previous results. We explore physical features of these parameters which (i) provide the scenario of complete evolution of the cosmos with $ω(t)$ and (ii) agree mostly with the observational status of the present phase of the accelerating Universe.

physics.gen-ph↗
arXiv2010

Dark Energy Models With Variable Equation Of State Parameter

Two variable $Λ$ models, viz. $Λ\sim (\dot a/a)^2$ and $Λ\sim ρ$ have been studied under the assumption that the equation of state parameter $ω$ is a function of time. The selected $Λ$ models are found to be equivalent both in four and five dimensions. The possibility of signature flip of the deceleration parameter is also shown.

gr-qc↗
arXiv2010

Higher Dimensional Dark Energy Investigation with Variable $Λ$ and $G$

Time variable $Λ$ and $G$ are studied here under a phenomenological model of $Λ$ through an ($n+2$) dimensional analysis. The relation of Zeldovich (1968) $|Λ| = 8πG^2m_p^6/h^4$ between $Λ$ and $G$ is employed here, where $m_p$ is the proton mass and $h$ is Planck's constant. In the present investigation some key issues of modern cosmology, viz. the age problem, the amount of variation of $G$ and the nature of expansion of the Universe have been addressed.

gr-qc↗
arXiv2009

Scenarios of cosmic string with variable cosmological constant

Exact solutions of the Einstein field equations with cosmic string and space varying cosmological constant, viz., $Λ= Λ(r)$, in the energy-momentum tensors are presented. Three cases have been studied: where variable cosmological constant (1) has power law dependence, (2) is proportional to the string fluid density, and (3) is purely a constant. Some cases of interesting physical consequences have been found out such that (i) variable cosmological constant can be represented by a power law of the type $Λ=3r^{-2}$, (ii) variable cosmological constant and cosmic string density are interdependent to each other according to the relation $Λ=-8πρ_s$, and (iii) cosmic string density can be scaled by a power law of the type $ρ_s=r^{-2}$. It is also shown that several known solutions can be recovered from the general form of the solutions obtained here.

astro-ph↗
arXiv2008

Scenario of inflationary cosmology from the phenomenological $Λ$ models

Choosing the three phenomenological models of the dynamical cosmological term $Λ$, viz., $Λ\sim (\dot a/a)^2$, $Λ\sim {\ddot a/a}$ and $Λ\sim ρ$ where $a$ is the cosmic scale factor, it has been shown by the method of numerical analysis that the three models are equivalent for the flat Universe $k=0$. The evolution plots for dynamical cosmological term $Λ$ vs. time $t$ and also the cosmic scale factor $a$ vs. $t$ are drawn here for $k=0, +1$. A qualitative analysis has been made from the plots which supports the idea of inflation and hence expanding Universe.

gr-qc↗
arXiv2008

The dark energy equation of state

We perform a study of cosmic evolution with an equation of state parameter $ω(t)=ω_0+ω_1(t\dot H/H)$ by selecting a phenomenological $Λ$ model of the form, $\dotΛ\sim H^3$. This simple proposition explains both linearly expanding and inflationary Universes with a single set of equations. We notice that the inflation leads to a scaling in the equation of state parameter, $ω(t)$, and hence in equation of state. In this approach, one of its two parameters have been pin pointed and the other have been delineated. It has been possible to show a connection between dark energy and Higgs-Boson.

gr-qc↗
arXiv2008

Generalized Model for $Λ$-Dark Energy

Einstein field equations under spherically symmetric space-times are considered here in connection to dark energy investigation. A set of solutions are obtained for a kinematical $Λ$ model, viz., $Λ\sim (\dot a/a)^2$ without assuming any {\it a priori} value for the curvature constant and the equation of state parameter $ω$. Some interesting results, such as the nature of cosmic density $Ω$ and deceleration parameter $q$, have been obtained with the consideration of two-fluid structure instead of usual uni-fluid cosmological model.

gr-qc↗
arXiv2007

Accelerating Universe with a dynamic cosmological term

Keeping in mind the current picture of an accelerating and flat Universe, some specific dynamical models of the cosmological term $Λ$ have been selected for investigating the nature of dark energy. Connecting the free parameters of the models with the cosmic matter and vacuum energy density parameters, it is shown that the models are equivalent. Using the selected models, the present values of some of the physical parameters have been estimated, and a glimpse at the past decelerating universe has also been presented. It is observed that most of these cosmological parameters nicely agree with the values suggested by the Type Ia Supernovae and other experimental data.

astro-ph↗
arXiv2007

Large Number Hypothesis: A Review

Large dimensionless numbers, arising out of ratios of various physical constants, intrigued many scientists, especially Dirac. Relying on the coincidence of large numbers, Dirac arrived at the revolutionary hypothesis that the gravitational constant $G$ should vary inversely as the cosmic time $t$. This hypothesis of Dirac, known as Large Number Hypothesis (LNH), sparked off many speculations, arguments and new ideas in terms of applications. Works done by several authors with LNH as their basic platform are reviewed in this work. Relationship between some of those works are pointed out here. Possibility of time-variations of physical constants other than $G$ are also discussed.

gr-qc↗